1. Field of the Invention
The present invention relates to infrared thermometers, more particularly to an infrared thermometer which uses an optical waveguide.
2. Description of the Prior Art
Sensing of infrared emission to measure temperature can be undertaken by one of many sensors known to the art, such as thermopiles, pyroelectrics, bolometers, and active infrared sensors. An infrared sensor generates an electrical signal which is representative of two temperatures. One is the surface temperature of the sensor T.sub.s, and the other is the temperature of the object or target T.sub.b. The relationship between these temperatures and the response of the sensor is governed by Stefan-Boltzmann law, EQU V=k.epsilon..sub.b .epsilon..sub.s (T.sub.b.sup.4 -T.sub.s.sup.4) 1
where V is the output signal of the sensor, .epsilon..sub.b and .epsilon..sub.s are emissivities of the target and sensor respectively, and k is a constant.
The ultimate goal of non-contact temperature measurement as in an optical infrared thermometer is to determine the temperature T.sub.b of the target. It is seen from equation 1, that to calculate T.sub.b, one must first determine two numbers, a reading V from the infrared sensor, and the surface temperature T.sub.s of the sensor.
The term "surface temperature" means a surface temperature of a sensing element positioned inside the sensor's packaging.
Obtaining the surface temperature of the sensor is not easy. An infrared sensor with a good response speed is generally fabricated in the form of a thin flake or membrane. The surface temperature is not only difficult to measure, but changes upon exposure to a target. Inaccurate determination of the surface temperature T.sub.s of a sensor results in an erroneous temperature measurement.
In order to overcome the problem, an alternative method of measuring temperature T.sub.b was developed. Instead of measuring the temperature of surface T.sub.s, temperature T.sub.a of a reference target is employed. Usually, measurement of T.sub.a can be performed with better accuracy. Therefore, equation 1 is modified to be: EQU V=k.epsilon..sub.b .epsilon..sub.s (T.sub.b.sup.4 -T.sub.a.sup.4)
In some inventions, such as the one described in U.S. Pat. No. 4,005,605 patented by Michael, the reference target is a cavity inside the thermometer. In U.S. Pat. No. 4,797,840, patented by Fraden, a fast moving shutter which occludes the sensor's field of view before measurement, serves as a reference target.
In any case, temperature of a reference target T.sub.a must be measured with high accuracy before it can be fed into equation 2 for calculating T.sub.b. Since that equation demands measurement of two independent variables V and T.sub.a, at least two sensors must be used in any infrared thermometer. One sensor is called the infrared sensor. It produces electrical signal V representative of the magnitude of thermal (infrared) radiation. The other sensor, often called the "ambient sensor", produces a signal representative of the temperature of a reference target T.sub.a which may come in one of many shapes and designs.
In many infrared thermometers and pyrometers, thermal radiation is measured by a thermoelectric device called a thermopile. In the above mentioned U.S. Pat. No. '840 Fraden, a pyroelectric detector in combination with a mechanical shutter is employed for that purpose.
In order to measure signal V, a definite and undisturbed volume of thermal radiation must reach the infrared sensor. The radiation is situated primarily in the far infrared (IR) spectral range. It must be channeled to the sensor by means of an optical system which is adapted to that specific range.
This invention is concerned with an element in the optical system path that channels infrared radiation between the reception portion of the thermometer and the sensor system in the body of the thermometer.
In a typical medical infrared thermometer which collects infrared radiation from a tympanic membrane and surrounding tissue within the human ear, the radiation is channeled by means of a waveguide which is a hollow tube with a highly reflective inner surface, as described in Fraden '840. Use of the reflective tube allows fabrication of a probe which can be inserted into the ear canal while keeping the infrared sensor and some other essential components such as the reference target, outside of the patient's body.
A reflective tube waveguide works like a mirrored channel in which light rays bounce from the opposite walls of the tube while propagating from one end of the tube to the other end of the tube.
For operation in the infrared range, the mirrored surface is made by polishing the interior of the tube, and applying a thin layer of gold, since gold is an excellent reflector in that spectral range.
For channeling IR radiation to the sensor in nonmedical applications in which the target is not as confined, the prior art teaches use of reflective focusing mirrors as in Michael '605, or lenses as in U.S. Pat. No. 3,586,439 patented by Treharne, or British patent 2 119 925 A, patented by Irani et al.
In measuring temperature in humans and animals, an infrared sensor cannot be positioned directly at the end of the probe. The probe has dimensions that are quite small since the probe should be inserted into an ear canal. In such thermometers, hollow tubular waveguides are presently employed almost exclusively.
There are several potential problems associated with use of a hollow waveguide. They include surface contamination resulting in loss of reflectivity, small but finite emissivity of the reflective surface resulting in stray emissions, a limited angle of view, and substantial signal loss in long waveguides having small diameters.
An approach which circumvents some of the above problems is taught in U.S. Pat. No. 5,167,235 patented by Seacord, in which infrared radiation is channeled to a thermopile sensor through a fiber optic bundle. One drawback of this approach is that optical fibers which operate in the far infrared spectral range are expensive and do not allow for controlling the field of view of the optical probe. This substantially limits use of fiber optic bundles.